1. Field of the Invention
This invention relates to a method for the heat treatment, in particular, annealing of a single crystal of II-VI semiconductors such as ZnS, ZnS.sub.x Se.sub.1-x, Zn.sub.y Cd.sub.1-y Se, etc. to dope with Group III elements as a donor impurity, and more particularly, it is concerned with a method for annealing a ZnSe bulk single crystal to be used for optoelectronic devices such as blue light-emitting diodes and laser diodes to decrease its resistivity.
2. Description of the Prior Art
As a method of decreasing the resistivity of a ZnSe single crystal, it has been proposed to obtain a low resistivity ZnSe crystal by subjecting the ZnSe single crystal to a heat treatment in molten Zn (J. Phys. D: Appl. Phys., Vol. 9, 1976, pp. 799-810).
However, there arises such a problem that in this heat treatment method, the ZnSe single crystal meets with increase of dislocation density and occurrence of cracks, thus resulting in marked deterioration of crystallinity thereof.
Following up this method, it is found that the dislocation density is increased from an order of 10.sup.4 cm.sup.-2 before the heat treatment to 10.sup.6 cm.sup.31 2 after the heat treatment.
As another method for obtaining a low resistivity ZnSe crystal, it has been proposed to charge a ZnSe single crystal and metallic Zn in an ampoule, followed by sealing, and then heat at a temperature of at least 1000.degree. C. in such a manner that the ZnSe crystal and metallic Zn are not directly contacted (Japanese Patent Laid-Open Publication No. 193700/1991).
When tracing this method, however, there arises such a problem that in the heat treatment as described above without adding a donor impurity, the crystallinity of the ZnSe single crystal is not deteriorated as compared with that before the heat treatment, but a very long time is required for obtaining a necessary low resistivity, and the resistivity, is only lowered to approximately 0.5 to 1 .OMEGA.cm with some dispersion of its value.
In the above described method, furthermore, when Zn vapor is condensed and solidified on a surface of ZnSe single crystal, stress occurs in an interface between them due to the difference in thermal-expansion coefficient of Zn and ZnSe single crystal, thus deteriorating the crystallinity of the ZnSe single crystal.
In the above described method, furthermore, the resistivity of the resulting single crystal largely depends on a very small amount of a donor impurity taken in during growth of the compound semiconductor and the amount of the donor impurity cannot be controlled by the heat treatment, so that the resistivity cannot sufficiently be lowered and the resistivity is largely fluctuated.
In the case of rapidly cooling, further, a large temperature gradient is caused in the ZnSe single crystal to deteriorate the crystallinity of the ZnSe single crystal. Even if a Group III element is used as a vapor source, its vapor cannot sufficiently be diffused in the ZnSe single crystal because of the lower vapor pressure and a desired resistivity cannot be obtained.